(WHTM) — Pennsylvania’s U.S. Senate race is being watched closely across the country with the balance of power in Washington D.C. potentially in play.

With polls showing the race tightening, there are some historic markers voters can keep an eye out for when the votes start coming in.

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Until 1914, Pennsylvanians didn’t vote for their United States Senator. The decision was up to party leaders and politicians until the 17th Amendment to the U.S. Constitution was passed.

Since voters cast their first ballots for a U.S. Senator in Pennsylvania, six races have come within 100,000 votes. Two of those races have come in the last four Senate elections.

  • 2016 (Toomey v. McGinty) 86,690 at 1.43%
  • 2010 (Toomey v. Sestak) 80,229 at 2.02%
  • 1994 (Santorum v. Wofford) 87,210 at 2.5%
  • 1964 (Scott v. Blatt) 70,635 at 1.5%
  • 1956 (Clark v. Duff) 17,970 at 0.4%
  • 1944 (Myers v. Davis) 23,684 at 0.64%

The closest Pennsylvania U.S. Senate race was in 1956 when Democrat Joseph Clark Jr. defeated Republican James Duff by 17,970 votes, or just 0.4%. It’s one of two races that finished with a margin of victory of less than 1% of the vote.

There have also been six races in Pennsylvania U.S. Senate election history in which the winner didn’t receive at least 50% of the vote. This is largely due to third-party voters, as evident in 2016 when it last happened.

In 2016, Pat Toomey narrowly won re-election when he received 48.77% of the vote, winning by less than 87,000 votes over Katie McGinty. Libertarian candidate Edward Clifford received 235,000 votes for nearly 4% in a presidential election year.

If Democrat John Fetterman should win in 2022, it would be just the third time Pennsylvania has had two Democratic U.S. Senators. It first happened in 1945 with Joseph Guffy and Francis Myers and last occurred when Arlen Specter changed his party affiliation to Democrat in 2009.

Before the 17th Amendment, there were Democrat senator pairs in the 1840s-60s.

The last time Pennsylvania had U.S. Senators from the same party was 2006 when Specter and Rick Santorum represented the Keystone State starting in 1994.

Election Day is Nov. 8, 2022.