By Jennifer Agiesta, CNN Polling Director

(CNN) -- Democrats hold an advantage in two states that are critical to the party's chances of taking control of the US Senate, according to new CNN polls conducted by SSRS. The surveys show Democratic Rep. Kyrsten Sinema and former Gov. Phil Bredesen leading their Republican opponents for open seats Arizona and Tennessee, where sitting Republican senators are retiring.

In Arizona, Sinema tops Republican Rep. Martha McSally by 7 points, 50% to 43% among likely voters, while in Tennessee, Bredesen holds a 5-point edge over Rep. Marsha Blackburn, 50% to 45% among likely voters there. Roughly 1 in 6 voters in each state say there's a chance they'll change their mind before Election Day.

Likely voters are a subset of registered voters in the poll and include those most likely to turn out based on a combination of self-reported intention to vote, interest in the election and past voting behavior.

Arizona and Tennessee are two of the four states where Democrats are widely seen as having at least some chance of picking up Senate seats in November's election. The others are Texas -- viewed as more of a long-shot - and Nevada -- generally viewed as the Democrats' best chance for a Senate pickup. In order for the party to have any shot at taking control of the senate, it's almost certain that at least one seat from Arizona or Tennessee would need to go Democrats' way.

Arizona has been a Democratic target for some time on account of its changing demographic profile, though the state hasn't voted for a Democrat in major statewide elections since Janet Napolitano's turn as governor in the Bush years. Tennessee has generally moved away from its more Democratic-friendly past. Those differences are readily apparent in the two states' impressions of President Donald Trump in the new polls. In Tennessee, likely voters are about evenly split on the president's performance, 49% approve and 48% disapprove, far outpacing his nationwide approval rating in the latest CNN polling of 36%. In Arizona, by contrast, Trump fares only slightly better than his national number, with 39% of likely voters saying they approve of the way he's handling his job while 57% disapprove.

The Republican incumbents for these seats -- Sen. Jeff Flake of Arizona and Sen. Bob Corker of Tennessee -- have both been publicly critical of Trump. Approval ratings of the president are closely tied to preferences in the Senate race, according to the poll. Among those voters who disapprove of Trump's performance in Arizona, 85% back Sinema, while in Tennessee, 92% of those who disapprove of the president back Bredesen.

Democrats hope that a ticket topped by Bredesen, the state's former governor whose positive favorability ratings outstrip the negative by a 2--to-1 margin (52% favorable to 24% unfavorable among the state's registered voters), can outweigh the state's underlying Republican tilt. Blackburn, by contrast, splits public opinion, with 41% of registered voters viewing her favorably and 39% unfavorably, with 20% unsure. Bredesen's edge here is driven by cross-party appeal. Although his favorability ratings are underwater among Republicans, 28% of them have a favorable view of him, while just 9% of Democrats have a positive view of Blackburn.

In Arizona, both Sinema and McSally are viewed more positively than negatively, though more than 2-in-10 likely voters say they have no opinion of each Senate candidate.

Health care tops the list of voters' most important issues in both states, with 29% calling it tops in their Senate vote in Tennessee and 25% saying the same in Arizona. The economy follows in Tennessee at 22%, immigration lands third at 16%. In Arizona, immigration is next on the list at 22%, with the economy just behind at 20%. Voters who say health care is their top issue are broadly supportive of the Democrat in both contests, breaking for Bredesen over Blackburn by 71% to 21%, and for Sinema over McSally by 75% to 14%. Both economy and immigration voters favor the Republican in each state. Blackburn holds a whopping 50-point lead among immigration voters in Tennessee and a 10-point advantage among economy voters. In Arizona, McSally tops Sinema by 33 points among immigration voters and 24 points among those who call the economy the most important issue in their vote.

In both states, Republicans have the upper hand in the gubernatorial race. Governor Doug Ducey narrowly tops David Garcia in Arizona, 49% to 46%, while Republican Bill Lee leads Karl Dean in the race for Tennessee's open governor's seat by 52% to 43%. Ducey's recent appointment of former Senator Jon Kyl to fill the Senate vacancy created by the death of Sen. John McCain earns high marks among Arizonans, 50% overall approve of the appointment while just 24% disapprove. Approval rises to 60% among those most likely to turn out to vote.

The CNN Polls in Arizona and Tennessee were conducted by SSRS September 11 through 15 among random statewide samples reached on landlines or cellphones by a live interviewer. In Arizona, results for the full sample of 1,001 adults have a margin of sampling error of plus or minus 3.8 percentage points, for the subset of 854 registered voters, it is plus or minus 4.1 and for the 761 likely voters plus or minus 4.3 percentage points. In Tennessee, results for the full sample of 1,000 respondents have a margin of sampling error of plus or minus 3.6 percentage points. It is 3.9 for the sample of 852 registered voters and 4.3 for results among the 723 likely voters.