WASHINGTON — Republicans who started off viewing Donald Trump as an amusing sideshow are starting to fret that the real estate billionaire is becoming the main event.

Since he defied skeptics and launched his presidential bid last month, Trump has rocketed in the polls, dominated media coverage and helped steer the debate on issues.

“I don’t know that he even knows how far he takes this,” former Rep. Thomas Reynolds (R-NY), who is close to GOP candidate and ex-Gov. George Pataki, told The Post. “He has the wherewithal . . . He has put together a pretty wholesale ground force in New Hampshire, and that has to be taken seriously.”

Trump accounted for a stunning 48 percent of all social-media and tra­di­tional-media conversation about politics over the last week, according to analytics group Zignal labs for The Washington Post. Trump had 1.9 million mentions, compared with just 448,000 for top Democrat Hillary Rodham Clinton.

Trump’s presidential rivals at first steered clear of his controversial comments about Mexican “rapists” pouring into the country — although several took opportunities in the last week to distance themselves from Trump. The pushback doesn’t seem to have hurt Trump, who continues to poll strongly and is assured a spot in next month’s Republican debate on Fox.

Trump told NBC’s “Today” show on Thursday he had “nothing to apologize for” and credits himself with raising the immigration issue in the campaign.

Trump’s success in early polling is undeniable.

He even leads the latest North Carolina poll, by Public Policy Polling, with 16 percent. He’s second to Jeb Bush in the latest CNN national poll and is also running second in Iowa.