For a moment last week, it looked as if Heidi Heitkamp might have caught a break - and from a surprising source, her U.S. Senate colleague Jon Tester, Democrat of Montana. Tester was the face of Senate opposition to Ronny Jackson, the president's choice to head the Department of Veterans Affairs. Trump vowed vengeance. Tester is one of 10 Democrats seeking re-election to the Senate in states that Trump carried. Heitkamp is another.

The political map of Montana resembles that of North Dakota, pretty much red all over, except in Indian Country and a few cities. Tester has won election twice, has never won handily, never managing to get more than 50 percent of the statewide vote. Heitkamp accomplished that in 2012, though just barely. Officially she had 50.24 percent, according to the secretary of state's website.

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Yet oddsmakers consistently rate Tester's chances of re-election higher than Heitkamp's.

Theoretically, Tester should be more vulnerable than Heitkamp. He's voted consistently against the president's appointees, including two high-profile choices, Neil Gorsuch for the Supreme Court and Mike Pompeo for secretary of state. Heitkamp voted to confirm both of them. Tester, therefore, should be a bigger target, and Trump's energy might be directed toward his race in Montana and away from Heitkamps's race in North Dakota.

A little reflection suggests that Heitkamp has more to worry about than Trump's endorsement of her opponent or even the possibility that Trump might spend time campaigning helping Rep. Kevin Cramer, who reluctantly agreed to make the race against Heitkamp after Trump told him, in effect, that he had to take one for the country. What Trump offered as inducements is unknown.

The attack on Tester highlights the real danger to Heitkamp's re-election bid. She's proven that she can draw crossover voters. Plenty of Republicans supported her in 2012, when she was initially elected. They helped her win, of course, but the real secret of her election victory was strong support from Democrats. She worked hard and her efforts drew more Democrats to the polls.

This year, Democrats have fewer incentives to vote. This is critical, since Heitkamp won by fewer than 3,000 votes.

Many of these votes came from Indian Country and the state's university communities. These, added to the party's usual base - a little less than a quarter of likely voters in the last several election - plus some Republican voters, put her over the top.

She might not be able to do that again, for three reasons.

One is that Native voters are disappointed in her for several reasons, but mostly her reaction to the DAPL protests. In 2012, Heitkamp won 80 percent of votes in the two most heavily Native counties, and she carried every legislative district that has substantial Native population. Native voters probably helped her in cities, too, giving her a near miss in Bismarck, for example, and helping her in Fargo and Grand Forks. It's impossible to know precisely how many votes were cast by Native people. My guess is between 8,000 and 10,000 - enough to make a difference.

They may not be unhappy enough to vote for Cramer, but they could stay home on Election Day.

New voter ID laws might influence voter turnout; Native communities are continuing a challenge of the law. Heitkamp also did well in inner cities, winning nearly two thirds of the vote

in District 18 in Grand Forks and District 21 in Fargo.

Another factor is at play here, an aging population. Like smokers, Democrats aren't being replaced.

It is therefore even more critical for Heitkamp to get every Democratic vote in this election than it was in 2012. It's also a bigger challenge. Democrats might be energized to vote against Trump, but perhaps not enough to support a Democratic candidate whose voting record is the most-pro- Trump among Senate Democrats. In effect, Heitkamp must appeal to Republicans while shoring up her support among Democrats. Cramer's only challenge is to hang onto his base and to draw back those Republicans who strayed in 2012, not because they didn't like the Republican Party but because they didn't like the party's candidate.

Winning elections is a little like baking bread. You can work the dough, but without yeast, it will never rise. The yeast in any campaign is an enthusiastic base of voters. This campaign, that's Heitkamp's most important voting bloc. She needs Republicans, of course, but she'll need more of them if Democrats stay home, and the more she appeals to Republicans, the more Democrats might do just that.

It's a tough calculation for the candidate, and success depends on getting it right. That's why Tester may be a better bet for re-election than Heitkamp. He's got the yeast provided by a loyal Democratic base. Heitkamp can't count on that, not based on her record or her campaign so far.