• Lamb has fundraising edge over Saccone.

With less than two weeks until a March 13 special election in Pennsylvania’s 18th Congressional District, upstart Democratic candidate Conor Lamb appears to have expanded his fundraising advantage over Republican stalwart Rick Saccone.

According to a report filed Thursday with the Federal Elections Commission, Mr. Lamb raised more than $3.3 million between Jan. 1 and Feb. 21. He spent $2.9 million during the same period, much of which covered the costs of TV advertising. But with over $400,000 already on hand at the start of the year, Mr. Lamb, of Mt. Lebanon, entered the final three weeks of the campaign with $837,408.82 in the bank.

Mr. Saccone, a veteran state representative from Elizabeth Township, raised $703,216.92 during the period and spent slightly more than $600,000. He went into the campaign’s homestretch with $303,034 on hand.

The money race isn’t over yet: While Thursday’s reports will be the last full accounting before the special election, both campaigns will continue to collect contributions in the contest’s final days.

Still, the fundraising momentum appears to have been with Mr. Lamb since the previous reporting period, which covered financial activity in 2017. In those early days of the special election campaign, Mr. Lamb led Mr. Saccone by a margin of roughly 2.5 to 1.

In the first seven weeks of 2018, meanwhile, Mr. Lamb has built up a fundraising advantage nearly twice that size.

For Republicans, the good news is that Mr. Lamb’s advantage is nullified, and then some, by the fact that Mr. Saccone has been able to draw on support from outside conservative groups including the National Republican Campaign Committee, Americans for Prosperity, and the Congressional Leadership Fund. Those groups have aided Mr. Saccone’s bid with mailings, TV ads, and paid doorknockers. As a result, Mr. Saccone still has the edge in areas such as TV exposure.

According to a review of Kantar Media data by political reporter Amy Walter, while Mr. Lamb’s campaign has put up nearly 2,000 TV spots to Mr. Saccone’s 345, outside spending has given Mr. Saccone the lead overall. Republicans of all stripes have run nearly 3,000 ads either bolstering Mr. Saccone or attacking Mr. Lamb, compared to slightly over 2,200 ads from Democrats.

In a statement anticipating the release of Thursday’s report, Mr. Lamb’s campaign noted that while Republicans had outspent him overall, “This grassroots fundraising has enabled us to compete and will enable us to win.”

National Republicans, meanwhile, have privately expressed misgivings about Mr. Saccone’s fundraising prowess. But Mr. Saccone has led in polling, albeit by narrow margins, and he shrugged off financial concerns.

“I’ve always raised the amount of money I need,” he told the Pittsburgh PostGazette on Thursday.

The fundraising momentum continues for Mr. Lamb. In the early days of the special election campaign, Mr. Lamb led Mr. Saccone by a margin of roughly 2.5 to 1.