In this photo taken April 19, 2018, campaign signs from both CD8 candidates adorn the main entrance sign to the city of Glendale in Glendale, Ariz. A special election is being held Tuesday to fill the seat left open by Republican Rep. Trent Franks, as Republican Debbie Lesko and Democratic candidate Dr. Hiral Tipirneni square off.

Democrats aim for another special election upset in a red area, this time in Arizona, but Republicans appear poised to avoid their second national debacle in two months.

Even if Republicans on Tuesday hold on to the House seat outside of Phoenix, Democrats hope they can add to a string of races in which they outperformed 2016 results in GOP-leaning pockets of the country. The ability to improve in red regions is critical for Democrats as they try to win the 23 GOP-held House seats needed to take a majority in the chamber in November.

Demographics and strong Republican leanings give the GOP a strong cushion in its effort to hold on to Arizona's 8th District House seat. The office was vacated when Republican Rep. Trent Franks — who held it since 2003 — resigned amid a sexual harassment scandal.

The election pits GOP state Sen. Debbie Lesko against Democratic former physician Hiral Tipirneni. Polling of the race has been limited and results have varied, but early voting patterns appear to favor Lesko.

While Tipirneni had raised more money and had more cash on hand than Lesko as of earlier this month, the Republican got a boost from national GOP groups trying to avoid another upset. Sensing a possible threat following a Democratic special election win in Pennsylvania, national Republicans injected some cash into the race.

Outside organizations including the National Republican Congressional Committee have spent about $740,000 supporting Lesko, while groups have put just under $50,000 toward opposing her, according to the Center for Responsive Politics. Meanwhile, outside groups have dropped more than $500,000 in anti-Tipirneni spending, compared with only about $230,000 spent to back her campaign. National Democratic Party groups have not gotten involved.