Montgomery County Commissioner Chairman Josh Shapiro has told national Democratic Party leaders that he will not run for the nomination to challenge Republican Sen. Pat Toomey next year.

"I am incredibly humbled and flattered by their interest, but ultimately concluded I am not interested in going to Washington to be a legislator at this stage in my career," Shapiro said Friday in a brief interview.

Senior Democrats had been encouraging Shapiro to jump into the primary against Joe Sestak, a retired Navy rear admiral and former Delaware County congressman who lost a close race to Toomey in 2010 but who also has a strained relationship with members of the party establishment.

Shapiro last week telephoned the three Senate Democrats who had been encouraging him - Minority Leader Harry Reid of Nevada; New York's Charles Schumer; and Montana's Jon Tester, the chairman of the national party's Senate campaign committee - to decline. He also informed Pennsylvania Democratic Sen. Bob Casey.

Democratic insiders say that Shapiro has been considering a bid for state attorney general in 2016. Embattled incumbent Kathleen Kane, a Democrat, is facing potential criminal charges in the leak of secret grand jury information embarrassing to a political rival.

Shapiro, 41, of Abington, is running for reelection in Montgomery County on November's ballot. He declined to comment on a possible campaign for attorney general.

Winning Pennsylvania is critical to the Democrats' hopes of retaking the Senate next year, when Republicans will be defending 24 seats. The state is reliably blue in presidential elections and with Hillary Rodham Clinton likely at the top of the ticket, Toomey could be vulnerable, the thinking goes.

But Sestak, 63, makes some party leaders nervous. It started when he ran against and defeated Sen. Arlen Specter in the 2010 Democratic primary. Specter had switched from the GOP after providing a crucial vote for President Obama's economic stimulus program in 2009, and Democrats from the White House on down wanted a clear field for the longtime incumbent.

Sestak lost to Toomey by 2 percentage points in the general election in a banner year for Republicans, fueled by the grassroots tea party revolt. That, Sestak supporters believe, proved his strength as a candidate.

He also goes his own way, preferring to take advice from family and old friends and acquaintances, and to hire Pennsylvania-based staff.

In such a targeted race, party officials in Washington typically want to have their most talented national operatives on the case.

Fund-raising also is a concern to some Democrats.

While Sestak has $1.7 million in the bank, he raised only $312,000 during the first quarter of 2015.

Toomey pulled in $2.1 million over the same period, and has $7.3 million on hand.

Marquee Senate races in big states such as Pennsylvania can cost upward of $30 million.

Allentown Mayor Ed Pawlowski announced on April 23 that he was running against Sestak for the nomination.

Pawlowski touts his record overseeing a downtown renaissance in the state's third-largest city, which is also Toomey's hometown.

Pawlowski briefly ran for governor last year but dropped out after raising just $346,000 in a race with no limits on campaign donations. Federal candidates are limited in how much they can get from individual contributors.

215-854-2718@tomfitzgerald