Sen. John McCain (R-Ariz.) fields questions from reporters on Capitol Hill on Feb. 15. (Mark Wilson/Getty Images)

We are living through another John McCain moment. The irascible, quotable war hero, just reelected to another six-year term to the Senate, has become once again a chief critic of President Trump.

At the Munich Security Conference, McCain — in the words of my colleague Aaron Blake — “systematically dismantled” the president's worldview. The cover of this week's New York magazine is “McCain vs. Trump,” a fresh profile by Gabriel Sherman in which the Arizona Republican says there must be investigations into Russian interference in our election, and looks gamely ahead to a growing resistance.

“One thing politicians look at are ratings, and his ratings are going to continue to decline,” McCain tells Sherman. “That means members of Congress will be more likely to resist things they do not agree with rather than roll over.”

McCain, who was personally insulted by Trump during the campaign and who pulled his support of him after the “Access Hollywood” tape was released, can command a lot of attention when he criticizes the president. For Democrats, who just lost an election that most people expected them to win, this appears to a boon. Historically, voters get concerned about where an administration is going if there is bipartisan opposition to its agenda. And historically, new presidents are not as unpopular as Trump, who, if not for the curiously buoyant Rasmussen poll, would be looking at average approval ratings below 40 percent.

But it's not necessarily good for Democrats when McCain leads a charge against Trump. In fact, it might represent a short-term problem.

Trump feeds off mainstream Republican opposition. Just as the president can't stop talking about how he won the presidency, we should not stop remembering how he changed party politics to do so. Trump smashed the mainstream consensus of political science that nominees need party elite support to succeed. Instead, he ran as a figure outside the normal party system, pulling in voters who did not consider themselves Republicans.

When The Washington Post published audio of Trump gloating about sexual assault, and some Republicans backed away from him — all of them expecting him to lose the election — Trump and his allies said this attitude was to be expected.

“This is basically the insiders versus the outsiders,” Trump adviser Rudolph W. Giuliani said at the time.

Trump has gotten remarkably far by portraying Republican opposition not as a sign of weakness but as proof that he — even in power — is feared by the nebulous “establishment.” Trump's closing campaign commercial was two minutes of him promising to smash the “globalists” who had weakened America. That's the sort of approach that largely inoculates him against — oh, let me see, let me pick an example at random — a speech by McCain at a conference in Munich.

Democrats need their base to see them resisting. In covering the (extremely long, but almost finished) race to run the Democratic National Committee, I've been struck by how cynical rank-and-file Democrats are about their party. The day after the Super Bowl, I covered a meeting between DNC candidate Thomas Perez and rural Wisconsin Democrats, and more than a few grumbled that the party was not doing enough to oppose Trump's Cabinet nominees. This was exactly when Senate Democrats were forming a solid, 48-vote bloc against education secretary nominee Betsy DeVos, and forcing — for the first time ever — a sitting vice president to bail out a Cabinet pick.

Pushed by their party's base, Senate Democrats have been moved from generally supporting Trump nominees to mostly opposing them. Ten Democrats up for reelection in 2018 come from states that voted for Trump — only two of them voted for Environmental Protection Agency pick Scott Pruitt. Yet the party's left is forever watching for a sellout. This meme from People for Bernie, a grass-roots group created in 2015 by veterans of Occupy Wall Street, illustrates it pretty well — that one Republican senator opposed Pruitt is seen as more telling than how 46 of 48 Democrats opposed him.

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As Democrats (finally) finish the race for DNC chair, there's already angst about the possible defeat of Rep. Keith Ellison (D-Minn.), who is backed by Sen. Bernie Sanders (I-Vt.). “Electing Tom Perez — which, insanely, is still very plausible — would be a grave misreading of the national mood,” Hamilton Nolan said this past week in Deadspin. “Worse, it would send a big flashing signal to not only Democrats but to everyone out there who is pissed that the Democratic Party is not taking this situation seriously.”

Perez, like Ellison, has opposed every move Trump has made. But right now, progressives view the Democratic Party warily. They can ill afford a story line in which Republicans like McCain, (or Evan McMullin, or Joe Scarborough) are the real leaders of the opposition.

Democrats need to oppose Trump on issues McCain doesn't talk about. Since the election, the Center for American Progress, Priorities USA and other progressive/pro-Democratic groups have conducted polling and focus groups to find out why voters bailed on them. The answer has been mind-blogging — voters in key states came to see Trump as the candidate who would smash “special interests,” and the Hillary Clinton-led Democrats as the party of the elites.

Many Democrats now see that problem as a creation of a misguided Clinton campaign, which chased after soft suburban Republican voters who could be persuaded to dump Trump. Clinton herself sometimes separated Trump from the larger GOP. “This is not a normal election,” she said last August. “The debates are not the normal disagreements between Republicans and Democrats.” The problem, as some Democrats worried at the time, was that Clinton was basically saying that the GOP led by House Speaker Paul D. Ryan — whose economic policies had been decisively rejected by voters when he was the party's 2012 vice-presidential nominee — was mainstream.

Ryan, right now, is critical of Trump's wilder comments, but generally happy with the opportunity for a “unified Republican government” to pass his agenda. Progressives and Democratic strategists alike now say they think the way to discredit Trump and Republicans is to argue that his Cabinet picks and agenda are selling out working-class voters. “This guy ran for president of the United States saying, 'I, Donald Trump, I'm going to take on Wall Street — these guys are getting away with murder,'" Sanders said earlier this month on CNN. “Then suddenly, he appoints all these billionaires.”

McCain has voted to confirm every Trump Cabinet pick save one — Office of Management and Budget Director Mick Mulvaney. He held out on him because of the former congressman's support for defense spending cuts. In the long New York magazine profile, McCain is not quoted criticizing any aspect of the Trump economic agenda. Democrats can criticize it all day — unless, of course, the story of opposition to Trump is one of a coalition of Republicans and Democrats opposing him over Russia and foreign policy.