THE MORNING PLUM:

Today voters go to the polls in Michigan and a handful of other states, and there is one week left until March 15th, when GOP primaries in Florida, Ohio, Illinois, and elsewhere will help determine the chances of stopping Donald Trump from winning the GOP nomination outright by vacuuming up a majority of delegates. If Republicans can do that, they might then try to block Trump at a contested convention.

Today’s Washington Post/ABC News poll brings Republicans who hope to stop Trump some good news, and some bad news.

First, the good news. The poll finds that Trump continues to lead among Republicans and GOP-leaning independents nationally: He has 34 percent; Ted Cruz has 25 percent; Marco Rubio has 18 percent; and John Kasich has 13 percent. But that represents a slight slippage in his lead; and he appears to be weakening on other fronts. Only a bare majority of 51 percent would be satisfied with Trump as the nominee, significantly lower than say the same about Rubio or Cruz. Over half give Trump the thumbs-down on a range of attributes, such as honesty, empathy, and temperament.

Also: Cruz beats Trump in a head-to-head matchup by 54-41, while Rubio beats him by 51-45.

Now, the bad news: Republicans who think Cruz would put the party in peril in November can’t be happy about the fact that Cruz does significantly better than Rubio does against Trump in the head-to-head matchup. It’s true that the map after March 15th is more favorable to Rubio than it is to Cruz, but such numbers will allow Cruz to argue that Rubio is the one who should drop out, and will likely encourage Cruz to stay in longer. And the poll also finds that a majority of Republicans oppose a contested convention:

As the ABC News team puts it, these numbers clearly indicate “the depths of divisions within the party.” And Trump is already laying the groundwork to exploit these divisions, should it come to this. Trump said this morning that if the nomination goes to someone else at a contested convention, “there are going to be a lot of people that will be very upset,” adding: “I think that would be pretty unfair.”

Remember, when Trump warns that treatment of him would be “unfair,” that’s code for: “Nice little party you got there. You don’t want anything to happen to it, do ya?” As Trump has said again and again, he signed that RNC pledge to support the GOP nominee on the condition that he is treated fairly. If he decides he hasn’t been treated fairly, all bets are off. And conveniently enough, Trump gets to decide what counts as fair or unfair treatment of him. So Trump could wreak all sorts of havoc, perhaps by urging his supporters not to back the GOP nominee. It’s obvious enough already that Trump doesn’t take kindly to losing.

The new Post poll also suggests a lot of Republicans who don’t back Trump might be unhappy with that outcome, though it’s hard to say what impact that would have.

Regardless, today’s voting and the voting on March 15th may determine whether denying Trump the nomination outright is even all that realistic a possibility. If Trump wins both Ohio and Florida, it will become very, very hard to prevent him from getting a majority of the delegates. As Benjy Sarlin puts it: “If Trump is not stopped in the winner-take-all states of Ohio and Florida on March 15, he will likely become the GOP nominee.”

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“It’s one thing if [Trump] goes to the convention and he’s got 48 percent, 49 percent of the delegates,” Tennessee Gov. Bill Haslam, a Rubio supporter, said in an interview here. “Then it’s a hard thing to see if there’s a convention floor battle. But if he goes to the convention and he’s got 35 or 40 percent, that’s a whole different thing.”

As noted yesterday, if all three non-Trump candidates stay in, it makes it more likely that Trump has a large delegate lead over the second-place finisher, even if he lacks a majority, making it perhaps harder to justify nominating someone else.

* THE FINAL FORECASTS: FiveThirtyEight gives Donald Trump a 92 percent chance of winning today’s Michigan Republican primary, and Hillary Clinton a greater than 99 percent chance of winning on the Democratic side.

The HuffPollster averages of Michigan polls find that Trump has 39 percent, to 21 percent for Ted Cruz, John Kasich 18 percent, and Marco Rubio at 14 percent. One thing to watch for will be how Rubio fares among groups that are supposed to give him an advantage in upcoming contests — younger, suburban, and better-educated Republican voters.

* TRUMP HOLDS NATIONAL LEAD: The latest NBC News/Survey Monkey Tracking Poll finds Donald Trump’s lead holding among registered Republicans nationally: He has 39 percent, Cruz has 20 percent, Rubio has 18 percent, and Kasich has nine percent. Rubio has slipped a little bit (three points), into third place.

Interesting tidbit: Despite Mitt Romney’s high profile speech blasting Trump as unfit for the presidency, and despite the expensive new GOP ad campaign to stop him, the percentage of GOP voters who think he’ll win the nomination has only gone up, to a new high of 75 percent.

* HILLARY’S LEAD SWELLS: On the Dem side, the new NBC/Survey Monkey tracking poll finds Hillary Clinton’s lead over Bernie Sanders has jumped to 17 points among registered Democrats, 55-38.

Noteworthy: Clinton is now nearly tied with Sanders among Democrats who are 25-34. While Sanders still holds a large lead among those under 25, this suggests she may be gaining ground with parts of a constituency that had proven elusive to her.

* A BIG BATTLE FOR SECOND PLACE TODAY: Nate Cohn and Jonathan Martin identify somethign to watch for in today’s Michigan results:

The race for second is a test for both Mr. Cruz and Mr. Kasich as each looks to present himself as the strongest Trump alternative. The second half of the primary season includes many Democratic-leaning states, like California, New Jersey and New York. A strong second-place showing in Michigan would ameliorate concerns about Mr. Cruz’s ability to compete in blue states; if Mr. Kasich were to fare better, it would set him up for a strong showing in his home state next week.

And so, today’s results could end up making it more likely that all the GOP candidates continue to stay in.

* KEEP AN EYE ON THE MARCH 15TH STATES: A new Chicago Tribune poll of likely Republican primary voters in Illinois finds that Trump leads with 32 percent, while Cruz has 22 percent, Rubio has 21 percent, and Kasich has 18 percent.

The outcomes in Illinois, Ohio and Florida on March 15th will determine whether Trump will have a serious shot at winning an outright majority of delegates, and with it, the GOP nomination.

 * AND THE SENATE IS AT RISK FOR GOP: Stuart Rothenberg explains why nominating Trump or Cruz would put GOP control of the Senate in danger:

At least five incumbent GOP senators from Democratic-leaning or competitive states were facing difficult re-election races this year even under the most favorable circumstances….Add in the deep division within the Republican Party, and the possibility of Trump or Cruz leading the national GOP ticket, and all – or at least almost all – of those races suddenly look much more uphill. In addition, states like North Carolina, Indiana, Missouri and Arizona look more interesting.