It’s interesting that Donald Trump still thinks that Joe Biden is his strongest possible Democrat challenger. I’m not sure he’s right about that, but it’s hard to dispute that the president believes it and that it informs his strategy.

President Trump is urging Republicans to vote in South Carolina’s Democratic primary on Saturday for the weakest candidate — determining that is Vermont Sen. Bernie Sanders.

Trump, hosting a rally in South Carolina on Friday night, asked his crowd to help him pick the recipient of GOP votes in the “open” primary.

“I assume this is okay from a campaign finance standpoint,” Trump joked before carrying out his “poll” on the eve of the primary election.

He asked audience members to cheer for the Democrat they’d prefer be his easiest-to-beat opponent — Sanders or former Vice President Joe Biden.

“I think maybe Crazy Bernie has it,” Trump said.

Asking one more time for his supporters to cheer for either former vice president Biden or Sanders, Trump confirmed: “They think Bernie’s easier to beat.”

I’m sure a few Republicans will follow his advice and vote for Bernie Sanders in the South Carolina primary today. Personally, my analysis says that this would be more likely to help the Republicans win House and Senate seats than to help Trump win reelection. From that standpoint, it could make some sense, but Trump isn’t known for his altruism toward his Republican allies. He must think Bernie would be easy to beat.

I think the current polls should at least cause Trump to reexamine that assumption. As I’ve said repeatedly, the problem with Sanders is less with how many votes he might pull but where he might pull them from. He’ll cut into Trump’s base while allowing Trump to make a comeback in the suburbs. This is likely to be damaging to House Democrats. Whether it will make it harder for Democrats to win statewide in some of the Senate and battleground states is harder to predict, since we have to know what the rural/suburban swap looks like, and also whether Sanders can uniquely boost youth turnout and add numbers that way.

Biden wins by crushing in the suburbs, doing less poorly than Clinton in the rural/small towns, and holding the maximum number of disaffected Republicans. He’s also uniquely strong in Pennsylvania, which is important. This kind of victory fits nicely with the Democrats’ House majority and their efforts to win Senate seats in places like North Carolina, Georgia, Colorado and Texas. But Sanders would win by bringing back more traditional Democrats and holding down Trump’s numbers in his strongholds by seriously competing for the youth vote there.

These totally different paths make a big difference for Congress but actually could be two ways of getting to the same place in terms of beating Trump.

Sanders’ path is more speculative and risky, and it will test the unity of the party. For that reason I don’t support going in that direction. But anyone who thinks Biden isn’t a risk hasn’t seen him campaign.