Mr. Tester’s top Republican challenger, Tim Sheehy, a wealthy businessman, announced that he had raised $2.5 million in the fourth quarter, including $450,000 of his own money. Republicans also expect Representative Matt Rosendale, a far-right agitator, to enter the race soon.

In Ohio, Senator Sherrod Brown, a Democrat also seeking his fourth term, raised $6.6 million in the fourth quarter and had $14.6 million on hand, according to his campaign.

Among the three top Republicans seeking their party’s nomination in Ohio, Bernie Moreno, a businessman known for his chain of car dealerships in the state, collected $800,300 during the fourth quarter and ended the year with about $2 million in cash on hand, his filing shows. Mr. Moreno, who has been endorsed by former President Donald J. Trump, spent $3 million on his own campaign last year and added another seven-figure check this month, according to a person familiar with the matter who was not authorized to speak publicly.

Another Republican candidate, State Senator Matt Dolan, raised $357,700 during the fourth quarter and started the new year with $4.8 million on hand, his reports showed. The third major G.O.P. contender, Secretary of State Frank LaRose, collected $812,500 during the fourth quarter and started the new year with $771,600 in the bank, according to his fillings.

Much of the suspense in the fight for the Senate has surrounded Ms. Sinema, however. Since leaving the Democratic Party in December 2022, she has been ambiguous about her plans for a second term, keeping the political establishment on a razor’s edge.