Senate Majority Leader Harry Reid (D-Nev.) with judicial nominee Goodwin Liu, and Sens. Diane Feinstein (D-Calif.) and Daniel Inouye (D-Hawaii) on Capitol Hill. (By Harry Hamburg/Associated Press)

Sen. Dianne Feinstein (D-Calif.) said Wednesday that Democrats, who would need seven Republicans to cross over in order to reach the 60-vote threshold, are expecting that the Liu vote will be a “very tight” one.

If the Senate rejects Liu’s nomination, it will mark the first time that Republicans have filibustered an Obama judicial nominee — nearly six years to the day after a bipartisan “Gang of 14” senators forged an agreement designed to prevent such filibusters except in “extraordinary circumstances.”

That means Thursday’s vote may be the end of the fight over Liu, but it also represents a turning point in the broader battle on judicial nominations.

As the first chapter in that struggle plays out on the Senate floor at 2 p.m. today, keep your eye on the following five senators:

Sens. Susan Collins (R-Maine) and Olympia Snowe (R-Maine): Of the senators who were part of the “Gang of 14,” nine remain in the Senate, including four of the group’s seven Republican members. Two of them – Sens. John McCain (R-Ariz.) and Lindsey Graham (R-S.C.) – said Wednesday that they will oppose Liu’s nomination. The other two – Collins and Snowe – have not yet indicated how they plan to vote. They’ll be under considerable pressure to explain whether or not they believe the liberal views many other Republicans have cited in their opposition to Liu rise to the level of an “extraordinary circumstance.”

Sen. Mark Kirk (R-Ill.): During his swearing-in ceremony last November, Kirk, a freshman who holds Obama’s former Senate seat, said he would be a “fiscal conservative, social moderate and national security hawk” in the Senate. So far, that’s proved to be the case; he was one of five Republicans in April to vote against defunding Planned Parenthood. If Kirk votes “no” on Thursday, look for him to make the case against Liu’s experience or judicial temperament, rather than against his ideology.

Sen. Jim Webb (D-Va.): Webb spoke out on the Senate floor Wednesday in opposition to Liu’s confirmation, citing concerns with some of the Liu’s views on issues including diversity and affirmative action. But Webb also noted that while he would vote “no” if Liu’s confirmation came down to a final up-or-down vote, he would not join Republicans in a filibuster on Thursday. If Thursday’s vote comes down to the wire, Democrats will be hoping that Webb doesn’t have a last-minute change of heart.

Sen. Dean Heller (R-Nev.): Heller, who as the successor to former Sen. John Ensign (R-Nev.) is the Senate’s newest member, faces his first controversial vote Thursday on a judicial nomination. His vote will be one of the first indications of where he intends to plant his flag within the Republican conference.

Some other members to watch – and who had not yet signaled their intentions as of Thursday morning – include Sens. Scott Brown (R-Mass.), Lisa Murkowski (R-Alaska), Dick Lugar (R-Ind.) and Lamar Alexander (R-Tenn.).