Donald Trump said his campaign would seek to join a bid by Texas to challenge the president’s election defeat at the U.S. Supreme Court. Now, 17 other states have filed in support of the case, which allies call the culmination of Trump's legal challenges and critics dismiss as a publicity stunt.

“This is the big one,” Trump tweeted Wednesday morning. Trump must petition the Supreme Court to be allowed to intervene.

Seventeen other states filed a brief in support of the Texas suit, brought by the state’s attorney general, Ken Paxton. Texas is seeking to prevent electors from Michigan, Georgia, Wisconsin and Pennsylvania from participating in the Electoral College on Dec. 14.

The Supreme Court on Tuesday evening gave the states until 3 p.m. Thursday to file responses.

Paxton’s case repeats allegations about mail-in voting that have already been rejected in dozens of courts across the nation.

The Texas suit was filed on the same day as the Dec. 8 “safe harbor” date for states to certify their slates of electors to send to the Electoral College. The passing of that deadline means time is short for Trump’s effort to overturn his re-election defeat, in which he’s sought to pressure state legislatures to override voters and appoint alternative electors who would back him instead of  Joe Biden.

In addition to Missouri, the states joining Texas were: Alabama, Arkansas, Florida, Indiana, Kansas, Louisiana, Mississippi, Montana, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Tennessee, Utah and West Virginia. All of the states were represented by Republican officials in the filing. All but three of the states have Republican governors.

Reuters contributed to this report.