Monday afternoon’s Senate vote is all about Democratic leaders finding another way around Max Baucus — one of the most frequent, unpredictable and enormously powerful thorns on their own side.

Senators will decide whether to break a filibuster helped by Baucus, who for years has been using his Finance Committee chairmanship to bottle up legislation leading to nationwide sales taxes on most online purchases. He says he can’t abide the measure’s effect in Montana — one of five states where there’s no sales tax, but where bigger businesses would have to collect sales taxes from Internet customers elsewhere. He says that’s both an unfair burden on his constituent businesses and an infringement on his state’s rights.

Baucus looks certain to lose; 75 senators voted for a nonbinding measure last month signaling support for the legislation. But the vote will also certainly do nothing to change the ways of a senator whose iconoclastic and parochially driven brand of centrism — especially when he’s within two years of an election — has often infuriated his leaders for the better part of two decades.

That’s because his approach has helped him repel a collection of vigorous challenges and win six terms in the Senate. It also makes him the front-runner at the moment to hold the seat again in 2014 even though President Barack Obama lost Montana by nearly 14 percentage points. Although his approval rating is at an underwhelming 45 percent, his $4.9 million in the bank at the start of April was more than anyone else in the “red state five” — the Democratic incumbents running next year in states Mitt Romney carried last year. And, although the recruiting of more formidable challengers hasn’t stopped, the only potentially viable opponent so far is a former Republican state senator, Corey Stapleton.

But it’s an axiom of Baucus’ congressional life that he’s only stayed safe by running scared, which helps explain why the Internet tax bill standoff marks the fourth time he’s so publicly scraped against the party grain in the past month.

Two of those times came just hours apart on Wednesday.

In the morning, he became the first senior congressional Democrat to publicly express apprehension about implementation of the health care law — which, of course, he had a central hand in writing, much to the consternation of his more liberal colleagues and many of the people in Montana.

“I just see a huge train wreck coming down,” he said, mainly when the enrollment period for the new insurance exchanges begins this fall.

“I don’t know what he’s looking at,” Health and Human Services Secretary Kathleen Sebelius snipped to reporters when the Finance hearing ended.

Then in the afternoon, Baucus and just three other Democrats broke with the party mainstream on all four key amendments to the gun violence bill. As the National Rifle Association wanted, he voted against expanding background checks, banning assault weapons and restricting high-capacity magazines, and in favor of allowing one state’s concealed-carry permit to apply nationwide.

His first high-profile apostasy of the year came just before the spring recess, when he was one of only four in his caucus to vote against the Democratic budget, which squeaked through without a single vote to spare. It calls on Finance to write a bill raising $975 billion in taxes in the next decade, which the chairman says is way too much. He slipped out of the chamber early in the roll call, even as Majority Leader Harry Reid was trying to figure a way to allow colleagues in even more pronounced political trouble the option of voting “no.”

The Nevada Democrat was reminded in all four instances that there is little percentage in expecting Baucus to put his personal political considerations behind his responsibilities as a top committee chairman to help his party with its legislative goals. Tom Daschle learned that a decade ago, when Baucus openly defied the previous Democratic leader’s orders to stay away from the negotiations that yielded the first Bush tax cut and the Medicare prescription drug program. In the 1990s, George Mitchell had to worry about Baucus’ balancing act pulling him away from the positions he was supposed to promote as Environment and Public Works chairman.

This spring’s gun control, Internet levy and budget resolution matters are tiny fare, though, compared with the No. 1 item on the Baucus agenda, which is to engineer the biggest tax law overhaul since 1980s.

Republicans eager for a Democratic partner who would see things their way — that the corporate and individual codes should be simplified in ways that don’t demand more taxes from the rich — are salivating at the chance to cut a deal with Baucus while he’s running for his seventh term. Many Democrats are openly leery of letting that happen and are counting on Obama to keep the brakes on a tax rewrite tamped down until 2015.

Baucus hopes then to break the record for time on Finance and, because his party doesn’t believe in term limits, to still be chairman. He will be 73 and presumably in his final term. And so it’s only then when his fellow Democrats think he might be willing to strike a deal entirely on his party’s terms.