There's a lot of debate happening now about whether the United States is heading toward a recession or whether we're already in one. And if some degree of downturn appears inevitable, how severe will it be?

That's of concern for investors as they ponder their portfolio picks: A major downturn could have a significant impact on economically sensitive businesses, cutting into their earnings, reducing their ability to make investments, and making it harder for them to cover their dividend payouts. 

Investors hoping to insulate their portfolios from a downturn may be on the hunt for some recession-proof stocks to buy now. If you're one of them, consider these three relatively recession-resistant companies that some Motley Fool contributors think would make great portfolio additions during these uncertain times: Mid-America Apartment Communities (MAA 1.67%), Bank of New York Mellon (BK 1.12%), and Realty Income (O 0.92%).

1. Checking the boxes for recession resistance

Marc Rapport (Mid-America Apartment Communities): When it comes to characteristics that make a dividend stock recession-proof, Mid-America Apartment Communities checks a lot of boxes.

One is that it offers a necessity-based product. Mid-America Apartment is a real estate investment trust (REIT) that owns and operates about 300 apartment communities spread across high-growth Sunbelt markets. That checks a second box, too -- having a large addressable market.

Another is the ability to raise rent or prices. Mid-America Apartment management said the REIT expects to end 2022 charging an average effective rent per unit that's 12.75% to 13.75% higher than it was a year earlier -- an inflation-beating growth rate.

A well-established leadership team is also a plus for steering an enterprise through hard times. Members of Mid-America Apartment's executive team have an average tenure of 21 years with the Memphis-based company. And over that time, they've delivered compound annual returns averaging about 15%, handily besting their residential REIT peer group and doubling the S&P 500 in terms of total returns over that time.

Total returns, of course, are a function of both share prices and dividends. Mid-America Apartment has raised its dividend payouts for 12 straight years and now yields about 3.3% at its beaten-down share price of about $151. Though the stock price is off by about 35% from its peak, the analysts covering it rate the stock a moderate buy, on average, and give it a consensus target price of $207.21

I own shares of Mid-America Apartment Communities and plan to regularly buy more for the stability and income it seems likely to continue providing, come what may on the economic front.

2. In rough economic times, fee income is your friend

Brent Nyitray: (Bank of New York Mellon): Bank of New York Mellon operates under a different model than the typical commercial bank or investment bank. It's a trust bank, which means that it holds securities on behalf of customers. Its typical customer might be a mutual fund, for which it will manage the investments and withdrawals, handle fund accounting, and provide other services. 

While commercial banks generally make most of their income from taking deposits and making loans, trust banks rely on fee income for a big part of their revenues. If the U.S. economy is headed into rough waters, then we should expect an increase in borrower delinquencies. That would require the big commercial banks to take provisions for credit losses, and eventually record write-offs, which would negatively impact their earnings. This makes commercial banks much more economically sensitive than trust banks. 

Bank of New York Mellon earned about 80% of its revenue from fees last year. Its main business is asset servicing, which includes custody services, fund accounting, and data analytics. It also has an investment services business that includes investment and wealth management. And it owns Pershing, a securities clearing firm for fixed income. Clearing firms match market trades between counterparties and ensure that when it's time to settle those trades, the sellers can deliver the promised securities and the buyers have the money. 

While Bank of New York Mellon is not as sensitive to economic conditions as most banks, it isn't immune to them either. The zero-percent benchmark interest rates that prevailed during the earlier stages of the COVID-19 pandemic were a headwind for it. The institution manages money market funds, and had to waive its fees on them in order to ensure that the investors in these funds remained whole. However, Bank of New York Mellon does have a competitive moat, and its stock should be a safe port in the storm if the economy takes a sharp turn downward. 

3. Built to withstand recessions

Matt DiLallo (Realty Income): Realty Income has a track record of growing regardless of how the U.S. economy is faring. The REIT has delivered earnings growth in 25 of its 26 years as a public company, with its only bottom-line contraction occurring during the Great Recession. Meanwhile, it has an unbroken 27-year streak of dividend hikes, putting it in the elite class of Dividend Aristocrats.

Its high-quality and durable portfolio of properties is a big factor contributing to Realty Income's consistent growth. The REIT gets 94% of its total rent from tenants in sectors that are resilient in the face of economic downturns and relatively isolated from the pressures of e-commerce -- businesses like grocery stores, pharmacies, automotive service centers, and warehouses. It utilizes long-term net leases under which the tenants are responsible for maintenance, building insurance, and even real estate taxes. These agreements often feature annual rental rate escalation clauses. This combination enables Realty Income to collect steadily rising rental income.

Meanwhile, the company has a strong financial profile. It has A-rated credit and a fairly conservative dividend payout ratio (for a REIT) of around 75% of its adjusted funds from operations. Because of this, it has the financial flexibility to expand its portfolio even during economic downturns. Realty Income acquired at least $1 billion of real estate each year over the last decade. These factors helped it achieve adjusted funds from operations per share growth of 5.1% annually, outperforming the sector average of 4% growth.

Realty Income has a proven ability to withstand recessions. Because of that, it is able to provide its investors with a steadily rising stream of dividend income. That makes it a great stock to consider buying if you're looking to recession-proof your portfolio.