A supporter crosses her fingers as she talks with Democratic U.S. Senate candidate Kyrsten Sinema at an event in Phoenix the Saturday before the midterm elections. (Bob Christie/AP)

Democratic candidate Kyrsten Sinema opened up a lead Saturday night over Republican rival Martha McSally in the U.S. Senate race in Arizona as officials count mail-in ballots, raising the prospects of Democrats winning a long-held GOP seat.

Sinema now leads McSally 49.51 percent to 48.15 percent, according to results provided by election officials at 7 p.m. Eastern time Saturday. The two congresswomen were separated by 28,673 ballots cast statewide, with a Green Party candidate lagging far behind. More than 2.1 million votes were cast.

The contest is to replace retiring Sen. Jeff Flake, one of the more outspoken Republican critics of President Trump.

Without any evidence, President Trump suggested foul play in the Arizona vote count in a tweet on Friday: “Just out — in Arizona, SIGNATURES DON’T MATCH. Electoral corruption — Call for a new Election? We must protect our Democracy!” the president tweeted.

In response, Flake tweeted: “There is no evidence of ‘electoral corruption’ in Arizona, Mr. President. Thousands of dedicated Arizonans work in a non-partisan fashion every election cycle to ensure that every vote is counted. We appreciate their service.”

Arizona is one of three U.S. Senate races that remain unresolved since Election Day. While Republicans are certain to control the Senate next year, the strength of their majority could be less than it appeared on Tuesday.

The Florida Senate race is headed to a recount, as the lead of Gov. Rick Scott (R) has narrowed to 12,562 out of nearly 8.2 million votes cast statewide in the race against Sen. Bill Nelson (D).

Meanwhile, a race for a U.S. Senate seat in Mississippi is headed for a runoff on Nov. 27. Appointed Republican Sen. Cindy Hyde-Smith faces Democratic challenger Mike Espy. Both candidates had about 41 percent of the vote on Tuesday.

As it stands, Republicans will hold at least 51 seats in the Senate come January, while Democrats will hold at least 46, including two that will be held by independents who caucus with the Democrats.

Depending on the outcome of the remaining three races, Republicans could hold as many as 54 seats, while Democrats could have as many as 49.

Republicans currently hold a 51-to-49 majority in the Senate.