You have to wonder whether credit-card companies really want to keep their customers.

Card issuers have slapped more than half of Americans with higher interest rates, lower borrowing limits, unexpected fees and late-payment penalties, according to a nationwide survey by the Consumer Reports National Research Center.

And credit card holders are angry. According to a national poll conducted by Consumer Reports, nearly one-third have paid off and closed a card since January 2008. Half of those that canceled did so in direct response to the actions of credit-card issuers, such as cutting limits, hiking rates, or imposing fees.

Twenty-one percent of respondents said they were treated unfairly by card companies, and only 41 percent said they were highly satisfied with their card issuers, making credit cards one of the lowest-rated services that CR covers.

The level of public anger about card issuers shows in the results of CR’s nationally representative survey of 1,211 credit card users, conducted in July 2009, as well as in scores of irate letters and emails CR has received from readers.

The survey also found that 45 percent of respondents say they are charging less, 43 percent say they are spending about the same, and 11 percent are charging more than they did a year ago.

The survey found that one-third of Americans say they don’t own a credit card. Of those Americans who use them, CR’s survey showed they tended to fall into three camps. One group (54 percent) is made up of consumers who generally pay their bills on time but use cards for convenience or to rack up rewards. Then there are those who reported moderate balances and reasonable prospects of eventually paying off that debt (33 percent of respondents). The third group (13 percent) includes consumers with debts totaling $10,000 or more, often from spending for emergencies; 44 percent of that group said they wouldn’t be able to survive financially over the next six months without relying on their credit cards to meet monthly expenses.

What can you do to protect yourself? CR offers strategies geared to three different types of users:

The on-time payer.

You make almost every payment on time, don’t carry a balance, and have a high credit score. You’re not concerned about your card’s APR because you don’t pay interest, but you enjoy earning rewards — cash back, points, or air miles.

What to do. Use rewards quickly, choose cash back over points or miles, and if you pay an annual fee for a rewards card, make sure you use it enough to make it worthwhile.

The low-balance cardholder.

You carry a moderate balance ($2,554 was the median in CR’s survey), and you always make your minimum monthly payments. But that balance, manageable with a single-digit interest rate, can become unaffordable when the rate goes up.

What to do. Roll over balances to cards issued by credit unions or regional or community banks. Credit cards from federal credit unions are capped at 18 percent APR, so even if you do fall behind on payments, you’ll avoid the 30 percent default rates some major cards charge. If you’re planning a purchase that you’ll need a year to pay off, consider switching to a card with a low introductory rate.

Users with growing balances.

Thirty percent of survey respondents who carried a balance said they owed more than $10,000. The median debt level in this group was $17,366, and 25 percent of people falling in this group said they had no idea when they might pay off the debt.

What to do. Negotiate with your card issuer. Thirteen percent of survey respondents said their issuer initiated offers of payment plans, reduced interest rates, or debt-settlement programs to get the balance paid off. If you feel overwhelmed, contact a nonprofit credit counselor through the National Foundation for Credit Counseling (www.nfcc.org).