Early voting surged in Harris County as more than 193,000 residents cast a ballot ahead of Tuesday's election, indicating Houston could see a spike in overall turnout.

Political scientists projected between 220,000 and 250,000 city voters will head to the polls by election night's close, up from more than 178,000 in 2009, the last time there was an open-seat mayor's race.

Friday marked the close of two weeks of early voting in Harris County.

Early turnout was particularly strong in African American and conservative areas, political scientists said, a boon to Houston mayoral candidates Sylvester Turner and Bill King.

"I think Sylvester could get close to 30 percent of the vote," Rice University political scientist Bob Stein said, noting that turnout by district so far "clearly advantages somebody like Bill King" for the second spot in a likely December runoff.

If those voting patterns continue through Election Day, the city's equal rights ordinance, dubbed HERO, also is expected to face a tough road to passage.

"This may spell doom or defeat for the HERO ordinance," TSU political scientist Michael Adams said, noting that turnout has been comparatively low among traditionally progressive inner-loop Anglo voters.

Citing a TSU analysis, Adams said about 53 percent of early city voters through Thursday were white, 28.5 percent were African American, 11.5 percent were Hispanic and 4 percent were Asian.

He also estimated that approximately 56 percent were Democrats, while 44 percent were Republicans.

As of September, more than two million Harris County residents were eligible to vote on Nov. 3, with more than 978,000 of them residing in Houston, according to the Harris County Clerk's office.

The share of votes cast early or by mail in recent mayoral races has increased steadily, from 28 percent in 2003, to 46 percent in 2013.

These figures do not include the handful of city precincts outside of Harris County.

Though some have speculated that this year's spike in early voting could portend low turnout on Election Day, Stein said he expects about half of those who cast a ballot will head to the polls on Tuesday.

Of the total early votes, 85 percent were in-person and 15 percent were mailed.

The Harris County ballot also includes several school and county bond referendums, as well as seven state constitutional amendments.

Early voting increased in neighboring Fort Bend and Montgomery counties, as well.

By Friday afternoon, 18,799 Fort Bend voters had cast ballots, an increase of 5,000 over 2013, said John Oldham, the county's election administrator.

"When we began this process, 10 percent (turnout) would have made me happy, but we're looking now at maybe 12 to 15 percent total," Oldham said.

Oldham attributed the increase in early voting to publicity surrounding the national presidential campaigns and voter interest in ballot issues, such as a $98.6 million facilities bond, an increase in the public school property tax exemption and municipal utility district propositions that would make monthly fire fees mandatory.

Oldham said he anticipates continued high voter turnout on Election Day, when Fort Bend will debut countywide vote centers. For the first time, Fort Bend voters will be able to cast their ballots from any of the 78 polling places in the county on Election Day.

"We could probably be looking at 50,000 people voting in this election, which is phenomenal for a constitutional amendment election, considering last year we were at 29,815," Oldham said.

With the last day of early voting still to be counted Friday, turnout already had climbed to 17,489, surpassing the number of Montgomery County voters who cast early ballots six months ago in the election that ultimately defeated a controversial $350 million road bond proposal.

Almost half the votes were cast in The Woodlands, which dominated the opposition to the last bond because it extended Woodlands Parkway, which some residents thought would draw more motorists into the heart of their community.

That $22 million project has been eliminated from the $280 million scaled-back proposal on the ballot.

Staff writers Cindy Horswell and Margaret Kadifa contributed to this report.