Until now, it seemed as if this year’s U.S. Senate race might have less in common with other contests for federal office and more with Mike Foster’s first gubernatorial election more than 20 years ago, with state Treasurer John N. Kennedy following in the footsteps of the two-term chief executive.

Not because Kennedy is also a former Democrat who switched parties, and not because both men have cultivated blunt-speaking personas. The relevant resemblance in this case is that, well before the 1995 nonpartisan primary, Foster shot to the head of a crowded field and stayed there, guaranteeing that he would snag one of runoff two spots. The only real question on election night was who Foster’s opponent would be, fellow Republican and former Gov. Buddy Roemer, or a Democrat, then-state Treasurer Mary Landrieu or then-U.S. Rep. Cleo Fields.

In the end, Fields edged the other two out, and pretty much everyone knew Foster would have an easy time dispatching him. Had Roemer or Landrieu grabbed the second spot, Foster might have looked a whole lot less invincible than he ultimately seemed.

Up until this week, pretty much every Senate poll has put Kennedy in that Foster slot, comfortably in first and facing several possible runoff opponents from both parties. But a new poll by Southern Media & Opinion Research suggests we don’t even know that much about the race’s shape less than two months out. The poll knocks Kennedy off his enviable perch, instead placing him just a hair ahead of another Republican, U.S. Rep. Charles Boustany, 16.9 percent to 15.2 percent, well within the poll’s 4.4 percentage point margin of error. Following behind are two Democrats, lawyer Caroline Fayard with 11.4 percent and Public Service Commissioner Foster Campbell with 9.2 percent.

And more than a quarter of the 500 voters interviewed Sept. 15-17 said they were undecided, so there’s no question that the race is in flux. Instead of one spot being basically taken, the poll suggests that both appear up for grabs, and there are still 20 more candidates in the race, some of whom are well known and could still rise into that top tier.

So why the shift?

That much is a matter of speculation. Kennedy remains popular statewide, with a 62.3 approval rating among the poll’s respondents.

But that doesn’t mean all those voters who like him envision him in Washington, despite his two previous runs for the federal office, one as a Democrat and one as a Republican.

It could be that they like him in his current job, where he has crafted an image as a tough-talking watchdog on state, not federal, budgetary issues. Boustany, a retired heart surgeon from Lafayette and Kennedy’s nearest philosophical rival for the votes of the more mainstream Republican voters, has only served in Congress, never Baton Rouge.

Or perhaps it’s because Kennedy doesn’t have a strong regional base, unlike Boustany, who has support from 54.1 percent of his own southwest Louisiana congressional constituents, the poll found. U.S. Rep. John Fleming of Minden, the next Republican in line and a leader in the confrontationally conservative House Freedom Caucus, is favored by just 24.9 of his constituents in the state’s northwest corner, which may well explain why he trails the race’s top two Republicans overall with 8.3 of the overall vote.

Or maybe it’s just that Kennedy has yet to start advertising on television, where Boustany has been running ads for weeks.

Whatever the reason, the poll suggests that any two of those top four has a good shot to make the runoff, particularly when you consider that about half of the voters who are likely to choose a Democrat have yet to settle on either Fayard or Campbell.

So far, the race has been relatively staid by Louisiana standards, except for the stray newsy development, from former Ku Klux Klan leader David Duke’s surprise entry to a new book raising sensational but unproven allegations about Boustany.

That was never likely to last, and the new poll suggests the action’s just getting started. In 1995, Foster was able to coast to primary and eventual runoff victory. This time around, everyone involved — and that now includes Kennedy — simply has too much to lose to be playing it safe.

Follow Stephanie Grace on Twitter, @stephgracela.