Considering how far left the political pendulum had swung during the elections of 1932, 1934 and 1936, the midterm elections of 1938 were probably never going to bring good news for the Democratic Party. Some degree of backlash was inevitable and with the GOP down to 88 members of the House and 18 senators, it’s hard to see how the Democrats could have possibly picked up more net seats in the midterms. It certainly didn’t help that there was rather extreme labor strife (involving both strikes and violence) during the summer of ’38, which was blamed by many on Roosevelt’s 1935 National Labor Relations Act. It also was problematic that the Depression economy dipped into recession that year and the unemployment rate went back up to twenty percent. However, Roosevelt had a more immediate and direct role in undermining his party’s chances. In March 1937, frustrated with rulings from a still conservative Supreme Court, he attempted to expand its membership so he could make additional appointments and win cases he had been losing. His effort failed, largely because of congressional opposition from his own party, but also because the plan was unpopular with the public. The failure hurt Roosevelt’s image, but his response had more far-reaching consequences.

In the summer of 1938, FDR intervened in certain Democratic primary elections, seeking to help engineer victory for liberal Democrats challenging conservative incumbents. His targets included senators Walter George of Georgia, “Cotton Ed” Smith of South Carolina, Millard Tydings of Maryland, Guy Gillette of Iowa, and U.S. representative John O’Connor of New York. These politicians had angered Roosevelt by opposing the addition of new justices to the U.S. Supreme Court, by refusing to go along with executive-branch reorganization, or because they said “no” to both. In each case, except for that of O’Connor, FDR’s opponent won. For President Roosevelt, the failure of the purge was a major setback. His attempt to establish a truly liberal Democratic Party, which would rescue the New Deal and propel it to greater heights, proved an embarrassment and a personal humiliation.

Several factors led to such a disappointing result. The White House “elimination committee,” composed of such stalwarts as Secretary of the Interior Harold Ickes and Works Progress Administration head Harry Hopkins, performed its job poorly. The committee never devised a coherent program for defeating Democratic enemies of the New Deal. Roosevelt himself was animated by personal dislike of Senator Tydings and perhaps some others, temporarily losing his compass. Most of all, the American people—particularly Southerners—did not want the president butting into state and local affairs. They rebuked Roosevelt, letting him know that he was out of his jurisdiction. The national Democratic Party and the state Democratic parties were not the same.

Here’s how Claremont McKenna College Prof. Andrew E. Busch describes the election result and the longterm repercussions:

When the election results were in, Democrats had lost six Senate seats and 71 House seats in what former Roosevelt advisor Raymond Moley called “a comeback of astounding proportions.” Republicans nearly matched the Democratic national House vote total, 47 percent to 48.6 percent; if one takes into account overwhelming Democratic predominance in the one-party South, the GOP clearly led the House vote in the rest of the country. Democrats also lost a dozen governorships, including such crucial states as Ohio, Michigan, and Pennsylvania.

Furthermore, Democratic losses were concentrated among pro-New Deal Democrats. Once the dust had settled, the Senate was about evenly divided between pro- and anti-New Deal forces, and the “conservative coalition” of Republicans and conservative Democrats was also solidified in the House, and started any given issue within range of victory. As political scientist David Mayhew has observed, the conservative coalition proceeded to dominate Congress for the next twenty years, until the election of 1958.

Political correspondent Arthur Krock held that “the New Deal has been halted; the Republican party is large enough for effective opposition; the moderate Democrats in Congress can guide legislation.” In addition, “the country is back on a two-party system… and legislative authority has been restored to Congress.” Republican spirits were revived, and the momentum of the New Deal halted.

The result in Congress was not a wholesale reversal of the New Deal but a stalemate in which Roosevelt was unable to make significant new departures, and indeed found himself in a defensive posture vis-à-vis Congress for the first time since assuming office. Congressional investigations began to embarrass the administration; Congress passed the Hatch Act (limiting political activity by federal employees) and Smith Act (cracking down on internal subversion) over FDR’s objections. For his part, Roosevelt offered no major new reform proposals in 1939 for the first time in his presidency.

If it makes sense to consider the 1930 midterm as the leading edge of the New Deal policy era, the midterm elections of 1938 clearly served as the endpoint of that era. Roosevelt was not rejected as Hoover had been—indeed he went on to win the next two presidential elections. But he never again dominated American domestic politics in the same way as before.

Prior to the 1938 midterms, the country was verging on being a one-party system and, as we still see in cities where the GOP cannot compete, when all the political action is within one party the tendency is for the really meaningful fights to happen in that party’s primaries rather than in general elections. When parties grow strong enough, they tend to try to purify themselves. How much this causes a revival of the opposing party and how much it is just something that generally accompanies a more generic “correction,” is something political scientists can debate. Maybe we can chalk it up to human nature, as something almost as inevitable as sunrise. In these circumstances, people will see little threat from the other party and focus instead on ideological battles within their own. However you calibrate this question, the failure of Roosevelt’s purge still provides a cautionary tale.

Some Republicans see or sense this, which is why many people both inside and outside the White House are worried about Donald Trump’s war on members of his own party. Recently, he has attacked Majority Leader Mitch McConnell as well as Sens. Lisa Murkowski, Susan Collins, Dean Heller, Lindsey Graham, and Jeff Flake. In the case of Flake, he has openly encouraged Kelli Ward, a declared primary challenger. His political patrons, the Mercers, recently donated $300,000 to a pro-Ward super PAC. And the president is traveling to Arizona today for unspecified reasons which many suspect will involve making his attack on Sen. Flake more explicit.

One thing should be obvious here. In 1938, there were 78 Democratic senators and 18 Republicans ones. Today there are 52 Republicans (one of whom is battling brain cancer) and 48 members of the Democratic caucus. The Republicans aren’t in a similar situation and should probably not act as if now is a safe or appropriate time to focus on purifying the ideology within their ranks. With enormous majorities, Roosevelt thought that would be a safe gamble and it wasn’t. How much less is it prudent to engage in that kind of internecine battle when your majority in about as narrow as it can get?

Roosevelt was a shrewd and experienced politician, and yet his effort suffered from a lack of coherency, plausibility and realism.  He wound up creating greater problems for himself and for the ideology he was defending.  Trump is not a shrewd and experienced politician, which makes it doubtful that his plan is more well thought out than the one devised by Harold Ickes and Harry Hopkins.

I have to agree with former communications director for the National Republican Senatorial Committee Brian Walsh, who says, “There are 10 Senate Democrats up for reelection in states the President won in 2016, and that’s where his political focus and energy ought to be over the next 14 months, instead of harmful intraparty warfare.”

Indeed, despite his troubles, the president should be encouraged by poll numbers that led Cook Political Report to downgrade the reelection chances last week of four Democratic senators: Heidi Heitkamp of North Dakota, Claire McCaskill of Missouri, Joe Donnelly of Indiana, and Joe Manchin of West Virginia.

If he were savvier and less divisive, he might have enlisted those four embattled senators in his efforts to put some legislative wins on the board. Failing that, he could focus on defeating them and expanding the GOP’s majority in the Senate. But he’s done neither of those things, which seems like political malpractice. Instead, he wants to exact revenge on members of his own party who are critical or who have cast important votes against him.

It might intimidate other senators and prevent them from sticking their necks out or withholding their votes, but it definitely did not work out that way for Roosevelt or for the liberals within the Democratic Party.

So, I say, “Please proceed, Mr. President.”