President Trump has tapped Neil Gorsuch to fill the late Antonin Scalia's seat on the Supreme Court. The Post's Robert Barnes tells you what you need to know. (Peter Stevenson, Gillian Brockell/The Washington Post)

There are plenty of flash points in the Senate fight over Judge Neil Gorsuch: President Trump's rocky start. The bitterness over the treatment of Judge Merrick Garland. But only one of them revolves around the Supreme Court nominee himself — the now-standard query looming in discussion of any high court candidate in recent years: Is he “mainstream” enough?

Defining your side's pick as “mainstream” is a familiar tactic in Supreme Court battles, one aimed at defanging the opposition's inevitable narrative that the nominee is an unhinged ideologue. It's also a slippery word to define — there is no legal definition of a “mainstream” judge. Here are some liberal Senate Democrats' interpretation of the exact same judge Republicans are praising:

“Unfortunately, Judge Gorsuch has proven to have a judicial philosophy outside of the mainstream,” said Sen. Kirsten Gillibrand (D-N.Y.) in a statement Wednesday.

Semantics don't necessarily affect the outcome of the fight — but they do have the potential to alter the battlefield terrain. Senate Minority Leader Charles E. Schumer (D-N.Y.) has said repeatedly since the election that his marker for supporting or opposing a justice will be how “mainstream” they are. "The need for a mainstream, consensus candidate is greater now than ever before," he said on the Senate floor Wednesday.

Republicans have the tools to push Gorsuch's nomination through without Democratic help, but they'd like to avoid a battle royal that ends up changing the Senate forever by blowing up the minority party's ability to block any nominee. So, they toss out the term "mainstream" to define him. In other words: Don't go to war over a judge who's an acceptable replacement to the conservative Antonin Scalia.

“I think the GOP is trying to say: 'You said you wanted mainstream. We gave you what we consider to be mainstream,' " said Molly Reynolds, a congressional analyst with the Brookings Institution.

Republicans' “mainstream” rhetoric appears to be specifically targeting some 10 red-state Senate Democrats who are up for reelection in states Trump won big, like West Virginia, Missouri, Indiana and North Dakota. Republicans are hoping to position Gorsuch in such a way that these more moderate Democrats could struggle to explain why they're joining a liberal-led filibuster of a Supreme Court pick that arguably any other Republican president would have picked, too.

"Sen. Debbie “Stabenow [D-Mich.] has said she has 'great concerns' about Gorsuch and how we need a 'mainstream pick,' " said Senate Republicans' campaign arm in a memo sent out Wednesday. “What does Stabenow have to say now that super liberal MSNBC host Rachel Maddow[e.nrsc.org] said Gorsuch was a 'relatively mainstream' choice for the court?”

The GOP is well aware that Senate Democrats are divided largely along ideological lines on whether they should filibuster Gorsuch. Progressive groups are demanding a full-stop blockade.

But it would only be the second time in modern history that a Supreme Court nominee has been filibustered, and the plan could backfire for Democrats by forcing Republicans to get rid of the ability to filibuster Supreme Court nominees — and taking away Democrats' ability to block a more conservative Supreme Court justice down the line.

Sen. Joe Manchin III (D-W.Va.), who represents a state Trump won by more than 40 points, issued a statement that appears to be leaning toward not going to war over Gorsuch, saying that he should get a vote and urging “my colleagues to put partisan process aside and allow the process to proceed.”

(Some Republicans are just as reluctant to change the rules, given that they cried bloody murder when Democrats did away with the filibuster for lower-court nominees in 2013.)

The battle to pitch Gorsuch inside or outside the mainstream is especially imperative in the context of the president who nominated him. President Trump uses terms like “extreme” to talk about vetting refugees. Twice in his first two weeks as president, there have been global protests against him or his policies.

Republicans seem to also be saying: If Trump is out of the mainstream, his Supreme Court pick is not. (Although some Democrats are attempting to undercut that argument by framing the Supreme Court battle less about Gorsuch's qualifications and more about Trump's divisiveness.)

In the end, Republicans will get Gorsuch on the bench. They'd just rather it be less of a fight than it threatens to be. Peeling away just enough red-state Democratic support with a candidate who's perceived as “mainstream” — whatever that means — could be their ticket to a much smoother nomination process.