Both parties are now racing to gauge the impact further down the ballot of a candidacy that could shatter traditional lines of combat in national politics.
Former Senator Norm Coleman of Minnesota, an influential Republican strategist and fund-raiser, said Mr. Trump’s nomination could imperil even the party’s seemingly iron grip on the House. Mr. Coleman said major donors were increasingly focused on building a “firewall” around Congress, because they believe Democrats would easily defeat Mr. Trump and keep control of the White House.
“Everything is in play: the presidency, the Supreme Court, the Senate and potentially the House,” he said.
Mr. Coleman said Republicans would have to decide on a case-by-case basis how to handle Mr. Trump in their races. “If it were me and I were running, and Trump were going to be at the top of the ticket, I would disavow him,” he said.
Republicans, who hold the Senate 54 to 46 and maintain control of the House by 30 seats, believe it would be essential for candidates running in diverse or comparatively affluent areas to break with Mr. Trump on matters of policy, and perhaps to denounce his nomination in blunt terms.
With control of the Senate resting in large part on Democratic-leaning states like Pennsylvania and Wisconsin, Republicans are preparing to run aggressively localized campaigns aimed at persuading voters to split their ballot for a Republican senator even if they support a Democrat for president.