Pittsburgh ? This coming Tuesday was supposed to be a big showdown in the Republican presidential nomination struggle, a titanic clash between economic and social conservatives that had all the elements of a breathtaking Pennsylvania primary. It was going to be a riveting confrontation featuring an insurgent and spurned native son against a party regular struggling to regain the sense of inevitability he thought was his birthright. Instead it’s the biggest dud since the Y2K scare.

The contest once known as Rick Santorum vs. Mitt Romney is over. Romney is now all but certain to become the Republican nominee in Tampa in August. But that doesn’t mean Romney’s struggles are over. In the next four months he still has to fight four battles:

Mitt Romney vs. the GOP House

One of the oddities of American politics is how both Democrats and Republicans are each two parties, a presidential party and a congressional party. This is always more evident when a party is out of the White House, as Republicans are today, and there is almost as big a chasm between Romney and House Republicans as there is between Republicans and Democrats on Capitol Hill, where the two warring sides at least have a shared experience and a shared argot.

To wit: The presumptive nominee already is finding himself pressured by his putative allies on Capitol Hill, and it comes in the form of impertinent remarks by freshmen lawmakers who were elected in districts of fewer than 700,000 people and have served in the House for fewer than 16 months.

Just the other day, a freshman Republican from Louisiana, Rep. Jeff Landry, asserted that he and his allies, not the likely GOP nominee, were the conductors of the Republican railroad, not necessarily the most felicitous legislative metaphor on offer in the Capitol. “We’re supposed to drive the train,” he said.

Imagine the young Lyndon Johnson, elected to the House in 1937, telling Franklin Roosevelt that the president was the porter and that he, Rep. L.B. Johnson of the 10th congressional district of Texas, was the conductor. At age 46, Johnson, the youngest Senate majority leader ever, began to emerge as a congressional tyrant, perhaps its greatest autocrat ever. At age 29, however, as a freshman member of Congress, Johnson was the sycophant’s sycophant — sickeningly so.

This phenomenon, a restive Republican congressional fifth column, is trouble for Romney — big trouble for him as a nominee, potentially calamitous if he is elected president.

Mitt Romney vs. the Santorum rump

They’re still there, the people who gave Santorum 11 state primary victories, more than half as many as Romney won. They still believe that Romney is a poseur, that he is not a genuine conservative, that he has no interest in, nor voice for, the social issues that propelled them into politics, that he is inauthentic and an opportunist. These things, mind you, aren’t being said by his general-election opponents, but by the people he will need in the November election.

The challenge Gov. Michael S. Dukakis of Massachusetts faced in 1988 to romance Rev. Jesse L. Jackson’s supporters was far less formidable than the one his gubernatorial successor faces in romancing the stone of the Santorum conservatives. Dukakis and Jackson disagreed on little and didn’t even have discernible differences in emphasis. They simply attracted different groups of broadly liberal supporters who opposed the Reagan-era economic and foreign-policy agendas.

Now add this to the calculus: Even with that advantage over Romney and facing the weakest Republican nominee in 14 years, Dukakis still lost 40 states to George H.W. Bush.

Mitt Romney vs. complacency

Romney has accomplished his heart’s desire, or at least part of it — a straight shot to the Republican presidential nomination that was his father’s fondest, but ultimately unrequited, desire. Nobody alive except former House Speaker Newt Gingrich thinks he has any real opponents left, and maybe even Gingrich couldn’t pass a lie detector test on that question.

Romney’s greatest opponent now is Romney himself. He is, to be sure, no shirker. But the interregnum between April and August is full of danger. It is a vacuum that Romney, who has no title and no job, must seek to fill in a positive way. It’s harder to do than it looks.

Mitt Romney vs. Barack Obama

This battle already has started, first on taxes, then on the question of stay-at-home-moms and working women. There is a geographical component as well. Both camps are establishing ground games for November.

The flurry around the notion that Obama thinks he has a shot in Arizona, home to two Republican presidential nominees in the last half century, symbolizes this new phase in the campaign — and the broader field on which it will be played. The Democrats have won Arizona only once since 1948 — and then, in 1996, by only about 2 percentage points. Discount 2008, when native son Sen. John McCain was the GOP nominee, and look at the 2004 election results: The Republicans won by 10.5 percentage points.

Not that that matters. If the Obama team decides to play in Arizona, investing staffers and advertising resources in a state where almost every indicator favors his opponent, it means that Romney will have to move campaign assets into a state where the Republican victory margin has averaged 167,495 votes in the last three elections.

Feints matter as much as offensives in presidential politics. And in Romney’s case, he is facing four challenges rather than one. He must win the first three if he is to prevail in the final one.