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With winter approaching, how about warming up your portfolio with some quality mid-cap stocks that could earn you good returns in the long haul? Mid-cap stocks are typically well-established companies with a market capitalization between $2 billion and $10 billion. While that saves them from the volatility that small-cap stocks suffer from, mid-cap stocks also usually possess greater growth potential than large-cap companies, making them a great investment option.

With that in mind, we asked three of our contributors to each pick a mid-cap stock they believe is worth buying this month. Find out why Ionis Pharmaceuticals (NASDAQ:IONS), Molina Health (NYSE:MOH), and RPM International (NYSE:RPM) made it to the list. 

Hedging the election

Todd Campbell (Molina Health): One mid-cap stock I'm watching like a hawk right now is Molina Health, a health insurer with big exposure to Medicaid, Medicare Advantage, and Obamacare that could benefit if Hillary Clinton wins the White House on Nov. 8.

Molina gets the majority of its sales and profit from Medicaid, and a Clinton win could lead to policies that support Medicaid enrollment growth. Clinton has also advocated for expanding Medicare, which could boost demand for Molina's Medicare Advantage products. And Clinton has said she'll improve the Obamacare marketplaces, rather than dismantle them, which could ultimately benefit exchange insurers, including Molina.

Changes to Obamacare that improve its viability for insurers could benefit Molina handsomely, because unlike commercial insurers that are losing big money on Obamacare, Medicaid insurers' low-cost infrastructure puts them in a far better position to profit from it. 

Molina's cost advantage over other insurers is significant. As a percentage of sales, SG&A is only 9% at Molina, but it's 14.8% at United HealthGroup, the nation's biggest insurer. Molina's advantage has it guiding for its ACA business to breakeven this year, and for ACA margins to improve in 2017. Currently, Molina serves 550,000 marketplace members, making it one of the 10 largest ACA insurers, and management is lobbying for "modification and adjustment, not wholesale change" to Obamacare, suggesting it sees the program as a long-term opportunity for upside.

Overall, Molina's potential to benefit from rising enrollment in government-facilitated health insurance programs means it's stock is worth considering this month, depending on the election's outcome.

This deep biotech pipeline is worth a look

Sean Williams (Ionis Pharmaceuticals): One top mid-cap stock that's had a miserable trailing month and looks potentially ripe for the picking is antisense drug developer Ionis Pharmaceuticals.

Before we dive into Ionis, you should know that this probably isn't an investment for everyone. Ionis Pharmaceuticals is largely a developmental-stage biopharmaceutical company that's likely to lose money throughout the remainder of the decade. The fact that it'll continue to lose money could constrain its valuation and makes it a very long-term play. However, for those of you with a long time horizon and a vision for Ionis' potentially game-changing drug-development platform, this could be an attractive entry point, with its stock down 29% over the trailing month.

Image source: Ionis Pharmaceuticals.

The near-term catalyst that could really ignite Ionis' valuation is nusinersen, a currently experimental spinal muscular atrophy (SMA) drug that dazzled in the phase 3 ENDEAR trial. An interim analysis of the study this summer showed that nusinersen met its primary endpoint for infantile-onset SMA. The positive results also triggered a $75 million licensing fee from development partner Biogen (NASDAQ:BIIB). If the drug is ultimately approved, Evercore ISI analyst Mark Schoenebaum believes it could generate peak sales of $1.7 billion within a decade.

Ionis may also benefits from the sheer size of its pipeline and the expanse of its partnerships. Ionis has a whopping 30 clinical trials currently under way, more than half of which are licensed to around a dozen different licensing partners. Having 30 clinical drugs gives Ionis multiple opportunities to hit a home run, while on the flipside ensuring that it never becomes too reliant on a single developing drug. The company's proprietary antisense drug platform, which utilizes past trial results as a predictor of future clinical effectiveness, also allows it to bring a good three to five new drugs into clinical trials annually.

Given its deep pipeline and the $672 million in cash it ended the second quarter with, Ionis Pharmaceuticals looks to be an intriguing top mid-cap stock worthy of your consideration.

A compelling stock for growth and income investors

Neha Chamaria (RPM International): If you find RPM International's business of coating, sealants, and specialty chemicals boring, you might've also heard how beautiful boring can sometimes be. In RPM's case, boring can be lucrative for you, especially after the stock's 13% drop in the past month despite a strong operational performance.

Last month, RPM reported a 13% year-over-year jump in net profits for its first quarter ended Aug. 31 and reiterated its outlook of 10%-12% growth in earnings per share for financial year 2017. That's incredible when you realize that the company's industrial segment -- which accounts for 50% of its sales -- is under considerable pressure because of currency headwinds and sluggish demand from heavy-equipment industries. RPM, however, has proved its credibility time and again by churning out solid profits and free cash flows year after year.

RPM Net Income (TTM) Chart

RPM Net Income (TTM) data by YCharts

So what do you get out of that stupendous FCF growth? Fatter dividend paychecks, of course! Having raised its quarterly dividend by 9% last month, RPM now boasts 43 straight years of dividend increases. So assuming that RPM will want to maintain its spot in the niche Dividend Aristocrat list, it should continue to pay you higher dividends irrespective of the business cycle. That certainly doesn't hurt, especially if you're a risk-averse investor.

Overall, RPM looks poised to weather the storm for years to come, and makes for an intriguing pick for both, growth and income investors.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.