Longtime Democrat Nancy Floreen launched an independent bid for Montgomery County executive on Wednesday, calling the Republican and Democratic nominees for the office “flawed extremes” and urging residents of the affluent and liberal suburb “to put principle and pragmatism above purely party politics.”

Montgomery’s 1 million residents have not chosen a non-Democrat to the top political job since 1974. But Floreen, an at-large council member, has been elected countywide four times. She could be a formidable opponent for Democratic nominee Marc Elrich, a progressive with strong union backing who also holds an at-large council seat and has supported rent-control laws and charging impact fees to developers.

“I am determined to give Montgomery County a third, independent choice come November,” Floreen said in a statement explaining her decision to drop her longtime Democratic affiliation to run as an independent.

Elrich narrowly defeated Potomac businessman David Blair in the June 26 Democratic primary, a six-way contest that was not decided until late Sunday after provisional and absentee ballots were counted.

Blair said Wednesday that “there has been an outpouring of support from our constituents asking us to request a recount.” He said he will decide whether to do so after the county Board of Elections verifies the primary results Monday.

Throughout the primary campaign, business leaders expressed concerns about Elrich, saying he would prioritize the social safety net and holding developers accountable over boosting economic development and improving the county’s business climate.

Elrich said in an interview Monday that he is not opposed to new development and has been portrayed inaccurately by his critics. He said he would work as county executive to expand the county’s commercial tax base to pay for programs such as expanding early-childhood education. He did not return calls for comment Wednesday.

Republican Robin Ficker is also on the ballot in the fall, and some Democrats worry that Floreen’s candidacy could split the Democratic vote and open a path to a Republican victory.

Of the 643,892 registered voters in the county as of June 9, nearly 60 percent are Democrats, about 17.5 percent are Republicans and 22 percent are unaffiliated with a political party.

Keith Haller, a political pollster and analyst who is based in Montgomery, said Floreen’s candidacy is a rare opportunity for independent voters to have a say in who becomes their top elected official, a decision usually made during the Democratic primary.

“This is an energizing moment for all those people in the county who feel disenfranchised or disillusioned by being excluded from county politics and government,” Haller said.

As Floreen gathers nominating signatures — she needs 7,200 by Aug. 6 — and campaigns for votes, she must navigate fissures in the party that came to the fore in the Democratic gubernatorial primary, when progressive former NAACP president Ben Jealous defeated Prince George’s County Executive Rushern L. Baker III, the establishment favorite, and four other hopefuls.

Although several prominent business leaders have said they will back Floreen, most Democratic elected officials in the county who have taken a position so far say they will support Elrich in the name of party loyalty.

“I am friendly with both. I thoroughly respect both,” council member Sidney Katz (D-Gaithersburg-Rockville) said of his two colleagues on the all-Democratic council. “But when you come through a primary, that is the right way to do it.”

Others supporting Elrich include Council President Hans Riemer and council members Roger Berliner (Potomac-Bethesda), who also sought the county executive nomination, Nancy Navarro (Midcounty) and Tom Hucker (Eastern County). Outgoing County Executive Isiah Leggett (D) and council member Craig Rice (Upcounty) did not respond to requests for comment.

Council member George L. Leventhal (At-Large), who also ran for county executive, said Monday that he has “always supported my party’s nominees,” but he stopped short of endorsing Elrich. State Del. C. William Frick, another county executive hopeful, said Wednesday he would “of course” support the Democratic nominee.

But former Rockville Mayor Rose Krasnow, who also sought the Democratic nomination for county executive, said she will back Floreen, whom she described as best positioned to lead the county at a time of tepid growth when, some say, businesses and residents are moving elsewhere because of high costs.

“I understand the idea that we should all support the Democrat, but I’m really worried about the future of our county,” Krasnow said.

The business leaders supporting Floreen include Bob Buchanan, chairman of the Montgomery County Economic Development Corp., who said this week that she is “a person the business community has come to rely on.”

Buchanan said Elrich “has an image of not being business-friendly or business-savvy.”

Montgomery County Board of Elections President Jim Shalleck said the board will vote on Floreen’s eligibility to run as an independent on Monday. Jared DeMarinis, director of the division of candidacy and campaign finance for the Maryland State Board of Elections, said this week that the state attorney general’s office had recommended that the board accept Floreen’s filing as long as she switches her party affiliation and collects the requisite signatures.

Robert McCartney contributed to this report.