POLL-Democratic challenger's lead appears to narrow in Arizona Senate race-Reuters/Ipsos poll

By Jason Lange

WASHINGTON, Oct 21 (Reuters) - Democratic U.S. Senate candidate Mark Kelly's lead over Arizona Republican incumbent Martha McSally appeared to narrow in a race that could determine whether Republicans lose control of the Senate, a Reuters/Ipsos poll showed on Wednesday.

There are about 12 competitive U.S. Senate races up for grabs in the Nov. 3 election, 10 with vulnerable Republican incumbents and two with vulnerable Democrats. To have a majority in the Senate, Democrats need to pick up three seats if the party wins the White House, which gives the vice president a tie-breaking vote, and four if not.

Here are the latest results for three Senate races on which Reuters/Ipsos is polling:

ARIZONA (Oct. 14-21 poll)

* Voting for Democratic challenger and former astronaut Mark Kelly: 51%

* Voting for Republican Senator Martha McSally: 44%

* Kelly was up 52%-41% in the prior poll.

* 27% said they had already voted.

NORTH CAROLINA (Oct. 14-20 poll)

* Voting for Democratic challenger Cal Cunningham: 47%

* Voting for Republican Senator Thom Tillis: 47%

* Cunningham led Tillis 46%-42% in the prior week.

* 12% said they had already voted.

MICHIGAN (Oct. 14-20 poll)

* Voting for Democratic Senator Gary Peters: 50%

* Voting for Republican challenger John James: 45%

* Peters led James 52%-44% in the prior week.

* 28% said they had already voted.

NOTES: The Reuters/Ipsos poll was conducted online and in English. The Arizona survey included 658 likely voters and had a credibility interval of 4 percentage points. The earlier Michigan poll surveyed 686 likely voters and had a credibility interval of 4 percentage points. North Carolina's surveyed 660 likely voters and had a credibility interval of 4 percentage points.

(Reporting by Jason Lange; Editing by Scott Malone, Cynthia Osterman and Jonathan Oatis)