The practitioners of politics at such rarefied heights are usually part of a relatively small universe: governors, senators, House members, cabinet secretaries, top elected state officials, operatives, advisers and big-money donors, among others. They tend to know or at least know of one another from years of rubbing shoulders at national and state conventions, myriad political dinners, campaigns, National Governors Association meetings, or wheeling and dealing in Congress. Even Ross Perot, who ran an independent campaign for president in 1992 with no elective experience, had broad dealings with the federal government over several decades.

As Mr. Alexander put it, “I have been to a lot of things over 40 years.”

That experience and track record provide those in the political world a working knowledge of whether candidates are true to their word, are willing to compromise, know the subject matter, can keep a confidence — all among the important things to weigh in making political judgments.

President Obama served only four years in the Senate, but even that short time gave his colleagues and critics some sense of how he operated and allowed him to build some solid relationships he has relied upon during his years in the White House. This year’s Republican primary included Senators Marco Rubio of Florida and Ted Cruz of Texas, who had both been in Washington a relatively short time but had been active in state politics, allowing people to form opinions based on firsthand experiences with them.

To Mr. Trump and his supporters, his lack of connection to establishment Republicans and the Washington influence community has been a singular strength of his candidacy, and he wears his lack of experience in public office as a badge of honor. As an outsider, he is seen by supporters as not weighted down by the loyalties and connections they view as a source of what is wrong with Washington.

But it could become a serious disadvantage if Mr. Trump is elected and needs to build a government. He has seemed to recognize his need for a running mate with Washington credentials and expertise, saying he wants someone capable of helping move legislation. And he did turn to a longtime Washington lobbyist and power broker, Paul Manafort, to help run his campaign.

Mr. Trump is not totally without acquaintances in Washington. Senator Harry Reid of Nevada, the Senate Democratic leader, and Representative Nancy Pelosi of California, the House Democratic leader, have recounted positive experiences with Mr. Trump.